DAP041 datasheet(29/40 Pages) ONSEMI

29 / 40 page

SCY99194

http://onsemi.com

Figure 56. Soft−Start Feature

Under some conditions, like a winding short−circuit for

instance, not all the energy stored during the on time is

transferred to the output during the off time, even if the on

time duration is at its minimum (imposed by the propagation

delay of the detector added to the LEB duration). As a result,

the current sense voltage keeps on increasing above VILIM,

because the controller is blind during the LEB blanking

time. Dangerously high current can grow in the system if

nothing is done to stop the controller. That’s what the

additional comparator, that senses when the current sense

voltage on CS pin reaches VCS(stop) ( = 1.5 x VILIM ), does:

as soon as this comparator toggles, the controller

immediately enters the protection mode.

In order to allow the SCY99194 to operate in CCM with

a duty cycle above 50%, the fixed slope compensation is

internally applied to the current−mode control. The slope

appearing on the internal voltage setpoint for the PWM

comparator is −32.5 mV/

ms typical for the 65 kHz version.

The slope compensation can be observable as a value of the

peak current at CS pin.

The internal slope compensation circuitry uses a sawtooth

signal synchronized with the internal oscillator is subtracted

from the FB voltage divided by KFB.

DAP041 Datasheet(PDF) 29 Page - ON Semiconductor